While peptide bonds usually adopt the trans conformation, peptide bonds to proline can exist in either cis or trans conformation. The isomerization between cis and trans is slow, and has been shown to be the rate-limiting step in folding of certain proteins.

What methods can be used to determine the conformation of a specific proline in a protein?

The authors report on the interactions between two SH3 domains of the Crk adaptor protein. Basically they find that the linker which tethers these two domains contains a proline which interconverts between the cis and trans conformations, and that this interconversion determines the interaction between the two domains. They follow isomerisation by using NMR.

Cis-trans peptidyl-prolyl isomerization is a subtle change, and I don't think that you will find a way of following it in a specific protein that is simpler than this, unfortunately.